The present invention relates to an organic electroluminescence device used as a light-emitting device for flat panel displays, projection displays, printers, etc., and a process for producing the organic electroluminescence device.
In flat panel display, a liquid crystal device has been widely used. Particularly, a so-called active matrix-type liquid crystal device (e.g., TFT (thin film transistor)-type liquid crystal device) wherein each pixel is provided with a switching or active element such as TFT has been used predominantly in the field of flat panel display.
In such an active matrix-type liquid crystal device, however, a nematic liquid crystal is generally used as a liquid crystal material and is accompanied with a longer response time (slower response speed) to an applied electric field, e.g., on the order of several ten milliseconds, thus being unsuitable for high-speed image display such as motion picture display. Further, the liquid crystal device is accompanied with a large dependence of viewing angle since a birefringence state of liquid crystal changes depending on a viewing direction.
In order to solve the above-mentioned problems, self-emission type devices for the flat panel display, such as a plasma emission device, a field emission device, and an electroluminescence device (hereinafter, referred to as xe2x80x9cEL devicexe2x80x9d) have attracted notice in recent years.
Of these self-emission type devices, the EL device is classified into an organic EL device and an inorganic EL device.
The inorganic EL device comprises a thin film EL device using an inorganic semiconductor (e.g., ZnS) driven by AC voltage application. The inorganic EL device is excellent in gradational characteristic and luminance but is accompanied with a problem such that the device is required to employ an AC drive voltage of the order of several hundred volts.
On the other hand, with respect to the organic EL device, T. W. Tang et al. have substituted in 1987 that it is possible to realize a high-luminance luminescence with low power consumption by utilizing a lamination structure comprising a film of fluorescent metal chelate complex and a film of diamine-based molecules.
The organic EL device is a self-emission device of carrier injection-type wherein electrons and holes are re-combined in a luminescence layer to cause luminescence (emission of light), as shown in FIG. 1.
FIG. 1 is a schematic sectional view of an embodiment of an ordinary organic EL device. Referring to FIG. 1, the organic EL device comprises a cathode 11, an anode 14, and an organic (compound) layer 16, including a luminescence layer 12 and a hole transport layer 13, disposed between the cathode 11 and the anode 14.
The organic EL device shown in FIG. 1 includes the organic layer 16 between the cathode 11 comprising a metal electrode and the anode 14 comprising a transparent electrode for emitting light therefrom. The respective layers (luminescence layer 12 and hole transport layer 13) constituting the organic layer 16 may generally have a thickness of the order of several hundred xc3x85. Examples of a material for the metal electrode (cathode) 11 may generally include those having smaller work functions, such as aluminum, aluminum-lithium alloy and magnesium-silver alloy. Further, as a material for the transparent electrode (anode) 14, it is possible to use an electroconductive material having a lager work function, such as ITO (indium tin oxide).
The organic layer 16 disposed between the cathode 11 and the anode 14 may have a three-layer structure including an electron transport layer 15, a luminescence layer 12 and a hole transport layer 13 as shown in FIG. 2, in addition to the lamination structure shown in FIG. 1.
The hole transport layer 13 has a function of efficiently injecting holes from the anode 14 to the luminescence layer 12. The electron transport layer 15 has a function of efficiently injecting electrons from the cathode 11 to the luminescence layer 12. Further, the hole transport layer 13 and the electron transport layer 15 also have functions of confining electrons and holes in the luminescence layer 12, respectively (i.e., carrier blocking functions), thus enhancing a luminescence efficiency.
With respect to carrier transport layers such as the hole transport layer 13 and the electron transport layer 15, it is important to improve charge (carrier) transport performances particularly a carrier mobility.
Generally, organic compounds in amorphous state have a carrier mobility of the order of 10xe2x88x925 cm2/V.sec, thus resulting in an insufficient (carrier) transport performance. Accordingly, if the carrier can be increased, it becomes possible to inject a larger amount of carriers into the luminescence layer, thus enhancing a resultant luminescence efficiency. At the same time, if a higher mobility can be achieved, it is possible to make a generally thin carrier transport layer (several hundred A) thicker (about 1 xcexcm), thus resulting in not only prevention of an occurrence of short circuit but also improvement in productivity.
For this reason, materials (compounds) for the carrier transport layers have been extensively developed in order to accomplish a high-efficiency organic electroluminescence device.
In such circumstances, some proposals have been made for achieving a higher (carrier) mobility by imparting a mesomorphism to an organic compound constituting a carrier transport layer (film). Generally, an organic layer (film) used in the organic EL device is in an amorphous state, thus having no regularity in molecular alignment (orientation). On the other hand, with respect to a liquid crystalline organic compound (or mesomorphic organic compound) exhibiting a certain order or regularity in molecular alignment or orientation, higher mobility materials have been found. Specifically, Haarer et al having confirmed that long-chain triphenylene compounds being a typical discotic liquid crystal exhibited a higher hole mobility of 10xe2x88x921 cm2/V.sec (Nature (1994), Vol. 371, pp. 141-). Haarer et al have also reported that a larger hole mobility was given by a higher degree of order in molecular alignment as a result of study on a relationship between a degree of order in molecular alignment and a hole mobility with respect to a series of triphenylene-based discotic liquid crystals in (discotic) columnar phase (Nature (1996), Vol. 8, pp. 815-). Further, Hanna et al have reported that a rod-shaped liquid crystal having a phenylnaphthalene skeleton exhibited a mobility of 10xe2x88x922 cm2/V.sec in its smectic E phase and that the liquid crystal had a high-speed bipolar carrier conductivity as to electrons and holes (Appl. Phys. Lett. (1998), Vol. 73, [25], pp. 3733-).
As described above, there is a possibility that molecular alignment advantageous to carrier transport is controlled by a spontaneous alignment (orientation) of a liquid crystalline organic compound, thus leading to a possibility that an excellent carrier transport material is realized.
Further, it has been found by our research group that it was possible to remarkably improve not only a carrier transport performance but also a carrier injection performance from an electrode based on a spontaneous alignment characteristic of a liquid crystalline organic compound as a result of study on the liquid crystalline carrier transport material (U.S. patent application Ser. No. 09/656,942), filed Sep. 7, 2000, corr. to Japanese Laid-Open Patent Application (JP-A) No. 2001-167888 and European Patent Application (EP-A) No. 1083613A2). Accordingly, it is possible to improve performances of organic EL device by using the liquid crystalline carrier transport layer as a carrier injection layer in combination with appropriate transport layer and luminescence layer.
The liquid crystalline organic compound also has an advantage in production process for an organic EL device. Generally, when an organic film is formed, vacuum vapor deposition is used for a low-molecular weight material, and spin coating is used for a polymeric material.
In the case where a thicker film (on the order of 1 xcexcm) of a low molecular weight liquid crystalline organic compound is formed between a pair of electrodes, it is possible to employ a process wherein the liquid crystalline organic compound is heated to isotropic phase temperature to increase its flowability and then is injected into a gap between the electrodes in isotropic phase state by utilizing the capillary action, or a bonding process wherein the liquid crystalline organic compound placed in its isotropic phase is disposed on one of a pair of substrates and the resultant substrate is applied to the other substrate to complete bonding of the substrates.
The thus prepared organic EL device may generally have a structure shown in FIG. 3.
Referring to FIG. 3, the EL device may include a pair of substrates 1 and 2, a cathode 11, an anode 14, an organic (compound) layer 16 (capable of having the structures shown in FIGS. 1 and 2), a liquid crystalline organic layer 17, and a spacer 18. On the substrate 1, the cathode 11 and the organic layer 16 (including the luminescence layer 12) are successively formed. The substrate 1 is disposed opposite to the other substrate 2 having thereon the anode 14 to leave a gap therebetween with the spacer 18. Into the gap, a liquid crystalline organic compound heated into its isotropic phase is injected, followed by cooling the form the liquid crystalline organic layer 17.
The thus prepared organic EL device allows self-emission of primary colors of red (R), green (G) and blue (B) by appropriately selecting materials constituting the luminescence layer. As a result, it becomes possible to provide a full-color panel with excellent performances, such as high-speed responsiveness and wider viewing angle compared with the above-mentioned liquid crystal device. In order to realize a full-color display with sufficiently practical performances, it is necessary to provide an organic EL device excellent in luminance, chromaticity and luminescence efficiency for respective luminescence elements of R, G and B.
Generally, in the case where each of respective luminescence layers of R, G and B is formed only of a single material, it is difficult to sufficiently satisfy luminance, chromaticity, luminescence efficiency, etc. In order to obviate the difficulty, a colorant-doped type organic EL device including respective layers each comprising a host material doped with a fluorescent organic compound (fluorescent colorant) as a luminescent center is utilized.
In such a colorant-doped type organic EL device, a material constituting the hole transport layer 13, the electron transport layer 15 or the luminescence layer 12 is used as a host material and the resultant layer is doped with a very small amount of a fluorescent colorant, thus causing luminescence of the color of fluorescent colorant to emit light of the color therefrom. As a result, it is possible to utilize a colorant having a high fluorescence yield, thus improving a resultant luminescence efficiency and allowing a more latitude in selection of respective luminescence colors.
When the organic EL device is used as a display panel, the device is required to be driven at a current and voltage as low as possible in view of the life thereof and a constraint on withstand voltages of drivers. In the circumstances, a driving method using switching (active) elements has been assumed to be suitable for the organic EL device (U.S. Pat. No. 5,786,796 corr. to JP-A 8-241057; 10 the Fine Process Technology for Flat-Panel Display Conference Proceeding E2).
In the case where a low-molecular weight type EL device using switching (active) elements each as means for supplying an electric field between a pair of electrodes is prepared, a transparent electrode requiring a high-temperature process is first formed on a substrate provided with a plurality of switching elements in view of heat resistance and thereon, necessary organic compound layers are successively formed by vapor deposition, followed by vacuum vapor deposition of, e.g., aluminum having a mall work function for forming a metal electrode to prepare an organic electroluminescence device. As a result, emitted light is generally observed from the transparent electrode side (i.e., the side of the substrate provided with switching elements).
Such an organic EL device is also expected to be applied to a device using a transparent plastic substrate and is promising device for paper display (CEATEC Japan 2000 Pioneer).
As described above, the liquid crystalline organic compound has excellent performances in terms of carrier transfer, thus being promising as a material for a carrier transport layer of an organic EL device.
Further, as described above, the liquid crystalline organic compound can be readily formed in a thicker layer according to the above-mentioned injection process utilizing the isotropic liquid state thereof exhibiting a flowability or the bonding process.
However, when the organic EL device is prepared by forming a carrier transport layer of an liquid crystalline organic compound in the above-described manner, a luminescence layer of the EL device is deteriorated to lower a luminescence or emission luminance in some cases.
Further, when the organic EL device including switching elements is prepared, light is emitted from the side of a substrate provided with the switching elements as described above. As a result, the presence of switching elements and wiring lines therefor leads to a lower opening rate (ratio) of pixels.
On the other hand, organic EL devices including a substrate provided with switching elements and a counter substrate and emitting light from the counter substrate side have been proposed in JP-A 11-3048 and JP-A 2001-35663. In these devices, however, there is still room for improvement in positional accuracy or stability when a substrate having thereon an EL element electrode and the counter substrate having thereon the switching element electrode are applied to each other so as to arrange pixels in prescribed position.
Incidentally, Tokuhisa et al have reported that polarizing luminescence is effected by an organic EL device wherein liquid crystalline organic layer as a luminescence layer is formed by injecting a liquid crystal compound into a gap between a hole transport layer and an electron transport layer (Applied Physics Letters, Vol. 72, pp. 2639-(1988)).
On the other hand, in the present invention, a liquid crystalline organic layer is utilized as a carrier injection (and/or transport) layer for injecting (and/or transporting) carriers into a luminescence layer.
A principal object of the present invention is to provide an organic electroluminescence device having solved the above-mentioned problems and a process or producing the organic electroluminescence device.
A specific object of the present invention is to provide an organic electroluminescence device capable of suppressing a deterioration of a luminescence layer by using a liquid crystalline organic compound which exhibits a high mobility and is readily formed in a thickness layer.
Another object of the present invention is to provide an organic electroluminescence device capable of improving a pixel opening rate by forming a lamination structure of a transparent electrode to be formed at high temperature and an organic (compound) layer having a poor heat resistance in a particular manner.
A further object of the present invention is to provide an organic electroluminescence device of the type wherein an organic EL layer and a liquid crystal layer are formed between opposite electrodes formed on a pair of substrates including one provided with switching elements so as to allow light emission (luminescence) from the side of the substrate free from the switching elements.
A still further object of the present invention is to provide a process for producing the above-mentioned organic electroluminescence device.
According to the present invention, there is provided an organic electroluminescence device, comprising: a pair of substrates each provided with an electrode, and an organic luminescence layer of an organic luminescence material and a liquid crystal layer of a liquid crystal material, respectively disposed between the substrates; wherein
the liquid crystal material has an isotropic phase transition temperature lower than a glass transition temperature of the organic luminescence material, and the liquid crystal layer has been formed by disposing the liquid crystal material on the organic luminescence layer in an isotropic phase state at a temperature lower than the glass transition temperature of the organic luminescence material and cooling the liquid crystal material to a temperature lower than the isotropic phase transition temperature.
According to the present invention, there is also provided a process for producing an organic electroluminescence device of the type comprising: a pair of substrates including a first substrate provided with a first electrode and a second substrate provided with a second electrode, and an organic luminescence layer and a liquid crystal layer, respectively disposed between the substrates the process comprising the steps of:
forming an organic luminescence layer of an organic luminescence material on a first electrode on the first substrate,
disposing a liquid crystal material on the organic luminescence layer in an isotropic phase state at a temperature lower than a glass transition temperature of the organic luminescence material, and
cooling the liquid crystal material to a temperature lower than the isotropic phase transition temperature thereof to form a liquid crystal layer.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.